1. Field of the Invention
This invention relates to a color-developing agent for pressure-sensitive recording sheets and a color-developing sheet which contains this color-developing agent.
2. Prior Art
Pressure-sensitive recording sheets are known as carbonless copying paper. They produce a color upon the application of a mechanical or impact pressure by writing or by pounding a typewriter, thus permitting duplication of several copies. The color is based on a color forming reaction between an electron-donating colorless dye and an electron-accepting color-developing agent.
As typical pressure-sensitive recording sheets there are transfer-type pressure-sensitive recording sheets and single sheet type pressure-sensitive recording sheet.
The transfer-type pressure-sensitive recording sheets are described in detail as follows.
The back surface of a tp (CB: Coated Back) sheet is coated with microcapsules having a diameter of several microns to ten and several microns and composed of a shell of a polymeric film such as gelatin, urea-formaldehyde resin and melamine-formaldehyde resin and of a solution of a colorless color-forming pressure-sensitive dye (leuco dye) in an involatile oil enclosed therein. The front surface of the bottom (CF: Coated Front) sheet is coated with a layer contaiing a color-developing agent having the property of reacting with the colorless dye upon contact therewith and thus producing a color.
The back surface and the front surface of the middle (CFB) sheet are coated with pressure-sensitive dye-containing microcapsules and color-developing agent on a base sheet, respectively. When a localized pressure is applied by a ball pen, typewriter etc. to pressure-sensitive recording sheets composed of a CB-sheet, a CF sheet and, if necessary, one or more CFB-sheets, so that the microcapsules-coated surface faces the surface coated with the color-developing agent-containing layer, the microcapsules under the applied pressure are ruptured and the solution of the carbonless dye moves to the color-developing agent-containing layer. Thus, the dye reacts with the color-developing agent to form a colored image in the desired pattern of recording.
On the other hand, in the single-type pressure-sensitive recording sheets, the pressure-sensitive dye-containing microcapsules and color-developing agent are coated as laminated layers or a mixed state-layer on the same surface of a base sheet.
Further, there is known a pressure-sensitive recording sheet, wherein the color-developing printing ink which contains pressure-sensitive dye-containing microcapsule and a color-developing agent as such or in capsule-form, is spot-printed on the required surface of a base sheet.
Still further, the color-developing agent which is dissolved in a solvent is used to check the state of the surface coated with dye-containing microcapsules. The color-developing agent of this invention is applicable for any uses as color-developing sheet, color-developing printing ink, color-developing solution, and so on.
The color-developing agents which are conventionally known include inorganic solid acids such as activated clay, attapulgite and so on (described in U.S. Pat. No. 2,712,507); substituted phenols and diphenols (described in the Japanese Patent Publication No. 9309/1985); P-substituted phenol-formaldehyde polymers (described in the Japanese Patent Publication No. 20144/1967); aromatic, carboxylic acid metal salts (described in the Japanese Patent Publication No. 10856/1974); 2,2'-bisphenol sulfone compounds (described in the Japanese Patent Laid-Open No. 106313/1979); and so on.
These known color-developing agent and the sheets coated therewith have both advantages and disadvantages. For example, inorganic solid acids are as advantage inexpesive and excellent in color-developing speed, but they have disadvantages that a color-developing ability deteriorates in a storage owing to the absorption of gases and moisture in the air, and the developed images discolor or fade prominently upon exposure to sunlight, fluorescent light etc. The substituted phenols provide insufficient color-developing ability and inferior image density.
P-substituted phenol-formaldehyde polymers (p-phenyl-p-phenolnovolak resin etc.) have superior color-developing ability, but have the disadvantage that the coating sheet undergoes yellowing upon exposure to sun light or gases in the air in th storage.
The aromatic carboxyic acid metal salts are superior in color-developing ability, light fastness of the colored image and resistance to yellowing under light, gas, etc. but their water-resistance and plasticizer resistance are not entirely satisfactory.